It is known that quadrature modulated subcarrier color television formats and systems are plagued in some picture conditions with a spectral overlap between the chroma subcarrier information and the high frequency luminance information with which the chroma information is interleaved. This spectral overlap has led the present inventor to examine carefully the subject of pre-processing a composite color signal at the camera or television studio or transmitter in order to minimize the spectral overlap prior to transmission. One example of the present inventor's prior work with adaptive encoding of quadrature modulated subcarrier color television signals is to be found in the method and apparatus described in U.S. Pat. No. 4,731,660, in the name of the present inventor and another. The disclosure of this prior patent (hereinafter "the referenced '660 patent") is hereby incorporated by reference into the present patent.
When a comb filter is used in the luminance path of a television color signal encoder in order to reduce cross-color effects after demodulation and decoding, there is a risk that the resolution of diagonal transitions in the picture display will be reduced. This risk has been minimized in the prior art by the use, for example, of the apparatus of the referenced '660 patent.
In the referenced '660 patent a method and system were described which carried out luminance comb filtering in response to a variable threshold derived as a function of at least one of luminance diagonal transition level, chroma level and averaged luminance activity in the signal spectrum in the vicinity of the color subcarrier. In other words, in the prior approach described in the referenced '660 patent, the frequency components which were otherwise creating cross-color artifacts were isolated from the composite picture and were then eliminated from the luminance path when, and only when, they were likely to result in cross-color artifacts upon decoding with conventional color decoders. The control was based upon adaptivity in the spatial domain, i.e. the measurement of the potential amount of cross-color damage and the generation of a control for eliminating the damage before it happened.
With prior art color decoders at the receiver, the method and apparatus described in the referenced '660 patent works well for most picture situations, but not for all picture situations. By prior art color decoders is meant decoders which have no comb filter luminance-chrominance separators, or which have very rudimentary comb filters based on a one scan line (1H) delay.
A drawback of the approach followed in the referenced '660 patent was that the described comb filter structure operated only upon information presented in the horizontal and vertical spatial domains. No motion control information was available from the temporal domain to control the operation of the comb filter structure. However, motion within the picture image has a direct impact upon the perception of cross-color.
One reason for following the method and implementing the apparatus described in the referenced '660 patent was the cost of implementation. Heretofore, temporal domain processing has been very expensive and has been implemented only in very expensive television systems. However, the cost of solid state frame storage devices is markedly declining to a cost level at which it becomes practical to include frame stores within both signal encoders and signal decoders of a color television system. Until the advent of relatively low cost temporal domain decoders, it was not very desirable or useful to implement time domain processing in the signal encoder.
Recently, NEC has proposed a receiver/decoder which uses time domain processing. In the NEC decoder chroma and luminance paths are separated by averaging over four fields (two frames). When four fields are averaged with plus coefficients, pure luminance is obtained. When four fields are averaged with minus coefficients, pure chrominance is obtained. However, this chroma-luminance separation process only works when there is no motion in the picture. When the picture moves, the averaging structure collapses. Thus, some attempts have been made by NEC, for example, to have motion adaptive decoding processes based upon motion detectors at the receiver. These processes essentially switch to the spatial domain when the time domain comb filtering process collapses.
In practice, this approach is not working very well because of the difficulty in distinguishing between motion and no motion conditions in the picture. It must be remembered that the color picture signal that is received is expected to be degraded and noisy, and it very frequently is degraded. The cumulative noise level, beginning at the camera and extending throughout the entire distribution path to and including signal processing elements of the receiver, introduces a very high threshold for motion recognition, which is above the cross-color generated by conventional encoding techniques and encoders. In other words, the temporal domain decoders, such as the NEC decoder using the IDTV approach in Japan, are collapsing in the presence of motion. The decoder works well when the picture image is stationary. When the picture moves, cross color and cross luminance artifacts become visible in the picture display.
Some improvement has been realized in Japan through use of encoders employing the spatial domain adaptivity techniques described in the referenced '660 patent. However, the approach being followed is perceived by the present inventor to be conceptually misdirected, and results in an unnecessary compromise in picture quality at the display. This compromised approach attempts to provide a full bandwidth when the picture image does not move by providing a very complex, and heretofore very expensive decoder including field delays. However, at the transmission end of the path, the encoder comb filter is usually operating, and by operating is reducing the bandwidth of diagonal transitions, a bandwidth the expensive decoder cannot not make up. Thus, for the new temporal domain decoders, a hitherto unsolved need has been for an encoder which is controlled by temporal domain activity, i.e. motion in the picture image, in a manner which is optimized for temporal domain decoders.
Another advantage for scene adaptive temporal domain encoding is that the appearance of cross-color artifacts in the picture display is much worse with moving picture elements than it is for stationary picture elements. Without motion, a luminance diagonal transition at 45.degree. will flicker at a constant 15 Hz rate in the NTSC signal format, creating chroma cross-color components which are changing phase at the 15 Hz rate. If the diagonal transition moves at a certain velocity, these chroma components become locked at a certain hue or color, such as solid red or solid green, for example. A solid color cross-color artifact is far more perceptible and is far more objectionable to the viewer than are the cross-color artifacts which merely flicker at 15 Hz, for example.
The reader skilled in the art will recognize that the new temporal domain decoders and the older spatial domain decoders are, to a certain extent, two non-compatible structures within the existing color television signal format, such as NTSC. While they are compatible in the sense of providing a color picture, the optimizations for the two decoders are not at all the same.
With spatial domain decoders of the prior art, it is very desirable to use adaptive time domain processing in the encoder structure. One optimization may be selected for spatial domain decoding and another optimization may be selected for temporal domain decoding. It is even more desirable to use scene-adaptive temporal domain processing within the encoder when it is known that a temporal domain decoder structure will be used at the television receiver/display. One optimization may be selected for spatial domain decoding and another optimization may be selected for temporal domain decoding.